The present invention relates to a method of producing a dual phase structure cold rolled steel sheet having low yield point, high elongation and tensile strength 35 to 50 kg/mm.sup.2.
Recently, automobile plants use high strength steel sheets in place of conventional mild steel sheets (SPCC and SPCD of JIS Standard) to comply with safety and weight reduction demands. However, stretchability of steel is decreased generally as strength is increased. Thus the high strength steel can not stand a high degree of press forming, so that availability of the steel is greatly limitted.
Recently, dual phase structure high strength steel sheet which is produced by continuous annealing is proposed to mitigate above mentioned disadvantage. The dual phase structure steel sheet is described in Japanese Layed Open Patent Appln. No. 39210/75 and U.S. Pat. No. 3,951,696. As shown in the patents, steel containing Si and or Mn is heated to .alpha. and .gamma. two phase range in constitutional diagram and is cooled relatively rapidly to obtain a structure having ferrite and transformation product and frequently involves retained austenite. The produced steel sheet has high tensile strength, low yield point and high elongation. In the above mentioned Appln. No. 39210/75, a steel containing 0.02 to 0.15% C, 0.7 to 2.870 Si, 0.7 to 2.5% Mn, and the ratio Si to Mn being between 0.6 to 1.5 with the balance being Fe and unavoidable impurities is cold rolled and is heated by desired heating rate, preferably 1000.degree. C. per minute. The steel is held at temperature 700.degree.-910.degree. C. for desired time, preferably less than 80 minutes and is cooled at cooling rate more than 100.degree. C. per minute. By such dual phase structure heat treatment, high strength and high ductile cold rolled steel sheet is obtained. In the U.S. Pat. No. 3,951,696, a steel containing 0.03 to 0.30% C, less than 0.7% Si, 0.6 to 2.5% Mn, 0.01 to 0.20 Sol Al, less than 0.015 O, less than 0.012% S with the balance Fe and unavoidable impurities is hot rolled and is cold rolled with a reduction more than 30%. The steel sheet is heated at average heating rate more than 3.degree. C. per second and is annealed for 1 to 15 minutes in a temperature range between A.sub.1 and A.sub.3 transformation points. Then the sheet is cooled at a cooling rate 0.5.degree. to 30.degree. C. per second as average cooling rate to 500.degree. C. High strength and high elongation cold rolled steel sheet is produced.
The above mentioned dual phase structure high strength steel sheet considerably widened applicability of high strength steel for automobile plants. The steel sheet has tensile strength more than 50 kg/mm.sup.2 and has low yield point and high elongation compared with high strength steel sheets of similar strength. However, the yield point is higher and the elongation is lower than those of conventional mild steel sheets. Thus, the dual phase structure steel sheets described in the above mentioned documents are not satisfactory to comply with severe demands for outer skin of automobile.